In electrical power distribution systems, several needs compete and must be simultaneously considered in managing electrical power distribution. A first concern has to do with maintaining delivered electrical power voltage levels within predetermined limits. A second concern relates to overall efficiency of electrical power generation and distribution. A third concern relates to these and other concerns in light of changing electrical loading of the system and variations in the character of the loading. A fourth concern relates to power system management under conditions associated with an increased probability of compromise of large scale ability to deliver appropriate power.
It is generally desirable to manage a power grid to reduce overall power consumption while maintaining adequate delivered voltage minimum and maximum levels across the system. In other words, the voltage levels actually delivered to various users need to be kept within predetermined limits while delivering power efficiently, without undue power loss in the delivery system or power grid, including the power generation equipment. As power usage within the system changes, in accordance with diurnal, weekly and seasonal factors, among others, need for regulation of power distribution changes as well. To an extent, some of these changes are reasonably predictable, however, other aspects of these changes may not be readily predictable.
Predictable changes in system loading are forecast by integrating power demand over time and considering this draw together with other factors, such as increased outdoor temperature and known diurnal variation patterns. For example, when summer heat results in increased power demand for air conditioning during the course of the day, fast food power demand associated with the end of the work day may indicate that a power shortage is imminent. Typically, measurements of power demand and delivered voltage are made every few seconds, filtered to reveal variations with periodicities on the order of a few minutes or longer, and adjustments to voltage are made perhaps once or twice an hour. This is called “conservation voltage reduction” and is intended to reduce overall power demand.
However, compromise of power delivery capability due, for example, to extreme weather conditions (e.g., gale winds affecting the distribution system) or unforeseen decrease in available power (e.g., generator malfunction) is not necessarily amenable to precise forecasting but is observable. As a result, there is need for dynamic system adjustment in response to observed changes in system capacity, conditions and loading.
Increased probability of compromise of large scale ability to deliver appropriate power may include increased probability of system-wide failure or blackout of an area, where “system-wide failure” could mean either a large grid being shut down or a smaller grid being isolated from a larger grid, with a potential result that the smaller grid then would be shut down or malfunction. In some cases, grid failure may be caused by automated shutdown of one or more generators in response to determination of grid conditions ill-suited to the generator in order to obviate catastrophic generator failure.
The conditions associated with an increased probability of compromise of large scale ability to deliver appropriate power are varied, and can range from “brownout” situations to complete disruption of electrical service or “blackouts”. Some types of power consumption relate to relatively vital concerns, such as hospitals, infrastructural support systems (telephone, police, fire protection, electrical traffic signals and the like) and others relate to more quotidian concerns, such as air conditioning, fast food operations and industrial operations such as aluminum smelters and the like, as equipment is added to or removed from service, for example.
The latter types of concerns can present a high electrical power demand at certain times of day. However, interruption of power delivery to such operations does not usually present life-threatening consequences when such operations are without electrical power.
Further, in the event of severe disruption or demand, grid systems used for delivery of electrical power can experience catastrophic failure when load conditions presented to generators in the system are such that one or more electrical generators are automatically shut down or disconnected from the system. This situation obviously places increased demand or even less suitable loading conditions on other generators or grids to which the grid is coupled. As a result, other generators or grids coupled to the affected grid may disconnect from the affected grid, potentially resulting in a blackout. Such blackouts can be extremely widespread in electrical generation and distribution systems employed multiple coupled grids each having electrical generation capability.
Electric utility distribution circuits are, generally, subject to both engineering and statutory constraints. For example, customers may expect very high availability of suitable AC voltage, so the electric utility distribution circuits should be reliable. Voltage limits may be specified for the voltage provided by the electric utility distribution circuits. In certain cases, exceptions to these limits also may exist, e.g., due to excess power demand conditions. The limits described so far may be codified by statute or regulation.
Other requirements also may be present, such as engineering requirements of the facilities. Electric utility distribution circuits may require consistency of application of circuit devices and materials in construction. Similarly electric utility distribution circuits may be required to operate consistently with cost effective sustainability, e.g., maintenance must be practical.
Electrical utility distribution circuits may benefit from various practices designed to improve efficiency or other performance metrics. For example, conservation voltage regulation (CVR) is the practice of reducing electrical energy consumption by operating electric distribution systems at voltages in the a lower portion of an allowable range, thereby improving the efficiency of many electric utilization devices. Many if not all utilization devices operate more efficiently in the lower portion of their designed voltage range. If those devices, motors, drives, electronic power supplies, transformers, lighting systems, etc. are applied properly, that is if they are not undersized for their application, virtually all will operate more efficiently.
In electrical power distribution systems as discussed above, one technique includes the use of an automated metering infrastructure (AMI). The application of automated metering infrastructure as the source of circuit voltage and demand metering in closed loop voltage optimization systems presents a computational problem for such systems. In the simplest case, a single distribution circuit (also referred to as a feeder) controlled by a single bank of voltage regulating transformers or by a single on-load tap changing transformer may serve thousands of electric utility customers. In the present context, each such customer site is assumed to be equipped with a metering device having voltage and demand metering capability, and telemetry capability such that its measurements may be observed by a voltage (or Volt/VAR) optimization system. The problem at hand is to select a subset of said metering devices that may be effectively used to inform the operational decisions of a closed loop voltage optimization system, subject to criteria derived from the performance qualifications of the voltage optimization system.